A, You need to just breathe and think about this rationally. If your H got those tickets with someone else in mind he certainly would have tried to hide it better, don't you think? If your H doesn't mind you opening his mail then he knew he risked you finding out about them.
Now that you know about the concert you can do some planning of your own. Any chance of having the kids spend that night elsewhere? Use your imagination and make it a night you'll both remember.
Do some things for your H in his love language to show him you really appreciate the special effort he put in in getting the tickets.
